Discovery of GAL3 Receptor Subtype and its Role
The investigations leading to the present invention arose from the discovery that mRNA for the GAL3 receptor is localized to areas of the rat brain associated with analgesia (see PCT International Publication No. WO 98/15570, published Apr. 16, 1998), thus supporting the expression of GAL3 in those regions. Protein for the GAL3 receptor is also shown to localize to areas of the rat brain associated with analgesia (see Table 12 and discussion herein).
This discovery led to the hypothesis that the GAL3 receptor may be modulating nociceptive information. Galanin is known to be released from the terminals of sensory neurons as well as spinal interneurons and appears to play a role in the regulation of pain threshold (Wiesenfeld-Hallin et al. 1992). In light of these reports, in vivo behavioral experiments were carried out to evaluate the analgesic properties of a selective GAL3 receptor antagonist. An animal model of neuropathic pain was employed to evaluate the use of selective GAL3 receptor antagonists to treat neuropathic pain. The Chronic Constriction Nerve Injury Model of Neuropathic Pain is a behavioral test that is used to assess the potential analgesic effects of compounds (Bennett an Xie, 1988). This model monitors the development of allodynia and hyperalgesia and is considered by experts in the field to reflect the potential of analgesic agents to treat neuropathic pain (Fisher et al., 1998; Fisher et al., 2002). This model is widely used as it is reliable across laboratories, and is sensitive to the effects of some of the major classes of analgesic drugs.
In an embodiment of the present invention the synthesis of novel pyrimidines which bind selectively to the cloned human GAL3 receptor, compared to other cloned human G-protein coupled receptors, as measured in in vitro assays, is disclosed. In a further embodiment of the present invention the synthesis of indolones which bind selectively to the cloned human GAL3 receptor, compared to other cloned human G-protein coupled receptors, as measured in in vitro assays, is disclosed. The in vitro receptor assays described hereinafter were performed using various cultured cell lines, each transfected with and expressing only a single galanin-type receptor.
From the binding information described hereinafter, it has unexpectedly been discovered that compounds which are specific for the human GAL3 receptor with a binding affinity greater than ten-fold higher than the binding affinity with which the compounds bind to a human GAL1 receptor are effective in animal models of pain which are predictive of efficacy in humans. Thus, we demonstrate that the GAL3 receptor antagonists, which may be classified as neutral antagonists, inverse agonists or allosteric modulators, provide a novel method to treat neuropathic pain.